Show captionMunira Wilson and Sarah Olney, the MPs for Twickenham and Richmond Park, cheer alongisde other campaigners outside the Royal Courts of Justice in London after the ruling. Photograph: Stefan Rousseau/PA

Plans for a third runway at Heathrow airport have been ruled illegal by the court of appeal because ministers did not adequately take into account the government’s commitments to tackle the climate crisis.

The ruling is a major blow to the project at a time when public concern about the climate emergency is rising fast and the government has set a target in law of net zero emissions by 2050. The prime minister, Boris Johnson, could use the ruling to abandon the project, or the government could draw up a new policy document to approve the runway.

The government is considering its next steps but will not appeal against the verdict. The transport secretary, Grant Shapps, said: “Our manifesto makes clear any Heathrow expansion will be industry-led. Airport expansion is core to boosting global connectivity and levelling up across the UK. We also take seriously our commitment to the environment.”

Johnson has opposed the runway, saying in 2015 that he would “lie down in front of those bulldozers and stop the construction”. Heathrow is already one the busiest airports in the world, with 80 million passengers a year. The £14bn third runway could be built by 2028 and would bring 700 more planes per day and a big rise in carbon emissions.

Johnson is thought to have been looking for a pretext to withdraw support for the extra runway and could make the argument for Birmingham to provide increased airport capacity for London given that train journey times will be reduced by HS2.

The court’s ruling is the first major ruling in the world to be based on the Paris climate agreement and may have an impact both in the UK and around the globe by inspiring challenges against other high-carbon projects.

Lord Justice Lindblom said: “The Paris agreement ought to have been taken into account by the secretary of state. The national planning statement was not produced as the law requires.”

Quick Guide

Heathrow third runway appeal court ruling - explained

What just happened?

For the first time judges have said that plans for a major infrastructure project are illegal because they breach the UK's commitments to reduce greenhouse gas emissions to tackle the climate crisis. This is a groundbreaking legal decision that could effect future infrastructure developments and puts the UK’s commitment to cut emission to net zero by 2050 at the forefront of future policymaking.

What will happen next?

The government has been told by the court of appeal to declare its decision to allow Heathrow airport expansion - contained in its airline national policy statement - illegal. Ministers have two choices now. They can withdraw the whole policy statement or try to amend it to make it compatible with the UK’s commitments to reduce greenhouse gas emissions to net zero by 2050.

Will the runway be built?

If the government can prove that expanding Heathrow is compatible with its commitments under the Paris agreement to very radically reduce greenhouse gas emissions, the runway may go ahead. But the prime minister has always been against the third runway, and the government has told the court it will not be appealing against its decision on Thursday.

There now hangs a very big question mark over whether the bulldozers will ever start work on the runway.

“It’s now clear that our governments can’t keep claiming commitment to the Paris agreement, while simultaneously taking actions that blatantly contradict it” said Tim Crosland, at legal charity Plan B, which brought the challenge. “The bell is tolling on the carbon economy loud and clear.”

Plan B’s intervention was one of a number of legal challenges against the government’s national policy statement, which gave the go-ahead for the new runway in 2018 after MPs backed it by a large majority. Others were brought by local residents, councils, the mayor of London, and environmental groups including Friends of the Earth and Greenpeace.

The challenges were dismissed in the high court in May 2019 but the complainants took their cases to the court of appeal, which delivered its verdicts on Thursday.

Plan B argued that the Paris agreement target, which the government had ratified, was an essential part of government climate policy and that ministers had failed to assess how a third runway could be consistent with the Paris target of keeping global temperature rise as close to 1.5C as possible.

“This is an opportunity for Boris Johnson to put Heathrow expansion to bed and focus on the most important diplomatic event of his premiership, the UN climate summit in Glasgow in November,” said Lord Randall, a former Conservative MP and climate adviser to the former prime minister Theresa May. “It’s his chance to shine on the world stage.”

The court of appeal did not overturn the high court’s dismissal of the other challenges, which related to air and noise pollution, traffic, and the multibillion pound cost of the runway.

But the Paris agreement ruling is far-reaching, according to Margaretha Wewerinke-Singh, an international public law expert at Leiden University, in the Netherlands. “Its implications are global,” she said.

“For the first time, a court has confirmed that the Paris agreement temperature goal has binding effect. This goal was based on overwhelming evidence about the catastrophic risk of exceeding 1.5C of warming. Yet some have argued that the goal is aspirational only, leaving governments free to ignore it in practice.”

Prof Corinne Le Quéré, at the University of East Anglia, said: “Government needs to put climate targets at the heart all big decisions, or risk missing their own net zero objectives with devastating consequences for climate and stability. I am relieved this is finally recognised in law.”

Climate campaigner Greta Thunberg said: “Imagine when we all start taking the Paris agreement into account.”

Heathrow and proponents of the third runway say it would provide an economic boost and is important for international business, particularly after Brexit. “The court of appeal dismissed all appeals against the government – including on ‘noise’ and ‘air quality’ – apart from one, [i.e. climate change] which is eminently fixable,” said a spokeswoman for Heathrow.

“We will appeal [as an interested party] to the supreme court on this one issue and are confident that we will be successful. Expanding Heathrow, Britain’s biggest port and only hub, is essential to achieving the prime minister’s vision of global Britain. We will get it done the right way.”

Mike Cherry, at the Federation of Small Businesses, said: “The verdict is a blow to small firms who need greater regional and global connectivity, as well as more opportunities to export.”

“No amount of spin from Heathrow’s PR machine can obscure the carbon logic of a new runway,” said John Sauven, at Greenpeace UK. “Their plans would pollute as much as a small country.”

Geraldine Nicholson, from local campaign group Stop Heathrow Expansion, said: “This is the final nail in the coffin for Heathrow expansion. We now need to make sure the threat of a third runway does not come back.”

At a separate event on Thursday, Alok Sharma, the business secretary and president of November’s UN COP26 climate summit, said: “The only economy which can avoid the worst effects of climate change, and thus continue to deliver growth, is a decarbonised economy. Our choices will make or break the zero-carbon economy.”

• This article was amended on February 28 2020. An earlier version had mistakenly called the business secretary Ashok Sharma, rather than Alok Sharma. This has been corrected.